Can't move question categories back to the top level

When editing question categories:

To reproduce
===========

1. Create a new 'Test' category - it defaults to having parent 'Top'.
2. Change the parent of the 'Test' category so that it becomes a child of the 'Default' category.
3. Try to change the parent back to 'Top'.

Expected result
============

The parent of 'Test' is set back to 'Top'.

Actual result
==========

'Test' stays under the 'Default' category.